Mankind has used geothermal energy for millennia. It is known that human tribes of the Neolithic Age bathed in natural hot springs, and the ancient Chinese and Roman civilizations built facilities to harness geothermal pools. With the core of the Earth believed to be over 5,000° C., it has been estimated that there is enough heat stored from the original formation of the Earth and generated by ongoing radioactive decay to meet mankind's energy needs for any foreseeable future.
The usual problems encountered in attempting to utilize geothermal energy have been practical ones of access, since the surface of the Earth is much cooler than the interior. The average geothermal gradient is about 25° C. for every kilometer of depth. This means that the temperature at the bottom of a well 5 km deep can be expected to be at a temperature of 125° C. or more. Oil companies now routinely drill for oil at these depths, and the technology required to create holes of this magnitude in the Earth is well known. (The deepest oil well at this time is over 12 km deep.) Wells of this depth, however, can be very expensive, costing over $10M to drill.
However, near geological fault zones, fractures in the Earth's crust allow magma to come much closer to the surface. This gives rise to familiar geothermal landforms such as volcanoes, natural hot springs, and geysers. In the seismically active Long Valley Caldera of California, magma at a temperature more than 700° C. is believed to lie at a depth of only 6 km. Alternatively, if lower temperatures can be utilized, a well dug to a depth less than 1 km in a geothermal zone can achieve temperatures over 100° C. A well 1 km deep often can cost much less than $1M to drill.
Electricity generation from geothermal energy was first demonstrated in Italy in 1904, but it was only in the 1950s that the first commercial operations began. The initial approach, such as that used at the Geysers facilities in Sonoma and Lake Counties, California, relies on natural steam within the Earth. At the Geysers, wells about 1-2 km deep penetrate the cap rock into a stratum containing magma-heated steam at a temperature of about 170° C. and a pressure of about 700 kPa (about 7 atm). The naturally high-pressure steam pushes to the surface through the well, and is directed to drive turbines to generate electricity. The water at the end is discarded as wastewater.
(For more, see <http://www.geysers.com/>.)
A more ambitious multi-year project in Iceland, the Iceland Deep Drilling Project (IDDP) along the mid-Atlantic ridge plans to drill wells 5 km deep to tap into a source of 500° C. hot supercritical hydrous fluid at about 220 atm in pressure.
(For more, see <http://iddp.is/about/>.)
Both of these projects tap into naturally existing geothermal pools of steam or superheated fluid. Such a system, often called a geothermal well, has its advantages in that the steam is naturally under pressure, and is replenished from a reservoir of groundwater. However, this technique can only be used in locations where there is magma nearer the surface to provide heat, where there is a steady supply of ground water to become pressurized steam, and a solid cap rock to keep the steam confined and under pressure. These conditions restrict the applicability of this method to relatively few geographic sites.
More recent methods to utilize geothermal energy in hot, dry rock are called enhanced geothermal systems, or EGS. [See The Future of Geothermal Energy: Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century, MIT Report, 2006, at <http://www1.eere.energy.gov/geothermal/egs_technology.html>.] In such a system all that is needed is for a pool of geothermal heat to exist at a depth where wells can be economically produced. In an EGS setup, a first well is drilled several kilometers deep and large volumes of water injected down into the hot rock. The water can be injected at temperatures that fracture the lower hot rock to make it more permeable. This process is called hydraulic fracturing, or “fracking”. The water being pumped into the injection well is then heated below the surface to become steam, and pumped out in a second well. This method for generating electricity is therefore similar to the previously described traditional geothermal technique, except in EGS the water is supplied by the system. The spent water, once the heat has been extracted to generate electricity, is re-injected into the injection well.
FIG. 1 illustrates a prior art EGS system. At or near the surface of the Earth 10, an EGS facility 12 provides a pumping system that injects water into the Earth and pumps water/steam from the Earth once heated. An injection well 14 extends into the Earth to a depth significantly hotter than the surface 10. The region of the Earth at this hotter temperature is designated a thermal pool 560. Water is then injected from the EGS facility 12 into the injection well 14, where it disperses into the thermal pool 560. Sometimes, the water is injected at such pressures that it causes a network of fractures 570 in the hot rock of the thermal pool 560, making it more permeable to water, and increasing the surface area of the rock in order to heat the water more quickly. Once the water is heated in the thermal pool 560, it is pumped out the production well 16, either as superheated water or as supercritical steam. The heated water/steam is used to drive a production facility 20 to generate electricity.
EGS can be used anywhere there is a suitable stratum of hot rock at accessible depths, as long as there is a supply of water to initiate the process and to replenish what is lost. Because the water/steam brought to the surface is intended to be recaptured once the heat is extracted and re-injected into the injection well, this is called a closed loop system. It is proving a popular alternative for geothermal energy, notably because it can be used in far more geographic sites than traditional geothermal wells.
EGS geothermal energy production facilities are being developed by several companies, including AltaRock Energy, Inc. of Seattle, Wash. AltaRock Energy Inc. has several issued patents on their technology, such as    U.S. Pat. No. 8,109,094 (SYSTEM AND METHOD FOR AQUIFER GEO-COOLING by S. Petty, filed Apr. 30, 2009 and issued Feb. 7, 2012); and    U.S. Pat. No. 8,272,437 (ENHANCED GEOTHERMAL SYSTEMS AND RESERVOIR OPTIMIZATION by D. Bour and S. Petty, filed Jul. 7, 2009 and issued Sep. 25, 2012); and has several applications pending, such as U.S. Patent Application    Ser. No. 12/432,306 (SYSTEM AND METHOD FOR USE OF PRESSURE ACTUATED COLLAPSING CAPSULES SUSPENDED IN A THERMALLY EXPANDING FLUID IN A SUBTERRANEAN CONTAINMENT SPACE by D. Bour, filed Apr. 29, 2009);    Ser. No. 12/433,747 (METHOD AND COOLING SYSTEM FOR ELECTRIC SUBMERSIBLE PUMPS/MOTORS FOR USE IN GEOTHERMAL WELLS by S. Petty, filed Apr. 30, 2009);    Ser. No. 12/538,673 (METHOD FOR TESTING AN ENGINEERED GEOTHERMAL SYSTEM USING ONE STIMULATED WELL by S. Petty, P. Rose and L. Nofziger, filed Aug. 10, 2009);    Ser. No. 12/754,483 (METHOD FOR MODELING FRACTURE NETWORK, AND FRACTURE NETWORK GROWTH DURING STIMULATION IN SUBSURFACE FORMATIONS, by S. Petty, M. Clyne and T. Cladouhos, filed Apr. 5, 2010);    Ser. No. 12/791,735 (SYSTEM AND METHOD FOR DETERMINING THE MOST FAVORABLE LOCATIONS FOR ENHANCED GEOTHERMAL SYSTEM APPLICATIONS, by S. Petty, O. Callahan, M. Clyne and T. Cladouhos, filed Jun. 1, 2010);    Ser. No. 13/326,285 (HIGH TEMPERATURE TEMPORARY DIVERTER AND LOST CIRCULATION MATERIAL by D. Bour, L. Watters, S. Petty and A. Apblett, filed Dec. 14, 2011); and    Ser. No. 13/342,924 (SYSTEM AND METHOD FOR AQUIFER GEO-COOLING by S. Petty, filed Jan. 3, 2012);which may be considered prior art for the invention disclosed in this application.
However, there are some drawbacks to such prior art systems using EGS. First, energy must be expended both to force water down into the injection well, and to pump the heated water/steam from within the Earth. Although the energy produced can still be significantly larger, it is an additional, ongoing cost. Second, EGS requires very large quantities of water to serve the needs of the injection well. In the western United States, the most likely area to deploy EGS because geothermal resources can be tapped with shallower wells, water is scarce and coveted resource. In those areas where sufficient water is available, additional problems arise due to the ultimate pollution of that water due to the minerals, salts and other toxic elements injection well water concentrates as it moves through the EGS cycle. Third, “Fracking” in the Earth at the bottom of the injection well can release methane, contaminating groundwater, and creates seismic events, which can sometimes be felt at the surface as earthquakes. A recent EGS project in Switzerland was suspended and ultimately cancelled due to strong seismic events (including a magnitude 3.4 earthquake) in the nearby city of Basel triggered by the injection well [see, for example, Domenico Giardini, “Geothermal quake risks must be faced”, Nature vol. 463, p. 293 (January 2010)].
FIG. 2 illustrates a prior art alternative approach to mining heat from dry hot rock as proposed by GTherm Inc. of Westport, Conn. In the prior art GTherm system, as in EGS, a surface facility 12-1 at the surface of the Earth 10 provides a pumping system 18-1 to inject water into the Earth through injection piping 14-1, and to pump water/steam from the Earth through production piping 16-1 once heated. However, in the GTherm system, a single well shaft 11-1 with a well head 15-1 extends into the Earth to the thermal pool 560, and contains both the injection piping 14-1 and the production piping 16-1. At the base of the well shaft, using underground drilling techniques such as potter drilling, developed by Potter Drilling Inc. of Redwood City, Calif. and described in part in U.S. Pat. No. 8,235,140 (METHOD AND APPARATUS FOR THERMAL DRILLING by T. Wideman, J. Potter, D. Dreesen and R. Potter, filed Oct. 8, 2009 and issued Aug. 7, 2012), a chamber 580 in the rock is formed surrounded by the thermal pool 560, and them sealed with a coating 590 of a special proprietary grout. This chamber 580 with coating 590 forms what GTherm designates a “Heat Nest”.
Water is then injected through the injection piping 14-1 into the chamber 580 with coating 590, creating a reservoir of liquid 550. This liquid 550 heats up, and is then pumped out of the same well shaft 11-1 through the production piping 16-1, either as superheated water or as steam. As in the previous EGS configuration, the heated water/steam is used to drive a production facility 20-1 to generate electricity.
This modified, single well EGS (SWEGS) closed loop approach of GTherm has some advantages over conventional EGS. First, once the heat nest has been formed, no fracturing of the bedrock need occur, meaning no seismic events will occur to disturb surface residents. Second, the water remains confined in the heat nest, and does not mix with local water sources or become contaminated with minerals or organic compounds from the local soil. Third, since the water used in the thermal loop does not mix with the local sources of groundwater, groundwater contamination does not occur unless there is damage or a leak to piping in the well shaft.
Several patent applications have been filed on this SWEGS technology, including U.S. Patent Application    Ser. No. 12/456,434 (SYSTEM AND METHOD OF CAPTURING GEOTHERMAL HEAT FROM WITHIN A DRILLED WELL TO GENERATE ELECTRICITY by M. Parrella, and filed Jun. 15, 2009; and    Ser. No. 12/462,656 (CONTROL SYSTEM TO MANAGE AND OPTIMIZE A GEOTHERMAL ELECTRIC GENERATION SYSTEM FROM ONE OR MORE WELLS THAT INDIVIDUALLY PRODUCE HEAT);    Ser. No. 12/462,657 (SYSTEM AND METHOD OF MAXIMIZING HEAT TRANSFER AT THE BOTTOM OF A WELL USING HEAT CONDUCTIVE COMPONENTS AND A PREDICTIVE MODEL);    Ser. No. 12/462,658 (SYSTEM AND METHOD OF MAXIMIZING GROUT HEAT CONDUCTIBILITY AND INCREASING CAUSTIC RESISTANCE); and    Ser. No. 12/462,661 (SYSTEM AND METHOD OF MAXIMIZING PERFORMANCE OF A SOLID-STATE CLOSED LOOP WELL HEAT EXCHANGER),all by M. Parrella and filed Aug. 5, 2009.
Although the SWEGS variation does offer improvements over conventional EGS, it still uses water as the fluid to carry heat from the thermal pool to the surface. As illustrated in Table I, if the temperature in the thermal pool is below 100° C., liquid water has a large energy density, and can do an efficient job of bringing heat to the surface. Water has a specific heat of 4.187 kJ/(kg ° C.) and a density of 1,000 kg/m3, giving an appreciable energy density of 4,187 kJ/(m3° C.). However, at a pressure of 1 atmosphere (1 atm, also 1.01 bar or 101 kPa) the temperature of liquid water is at most 100° C., and therefore the amount of heat that can be raised with each kilogram of water is limited by its boiling point.
TABLE ITable I: Specific Heat, typical Mass Density, and EnergyDensity of water, steam, and various other substances.SpecificMassEnergyHeatDensityDensitykJ/(kg ° C.)kg/m3kJ/(m3 ° C.)Water (20° C.)4.1871,0004,187Superheated Water8.1385794,712(161 atm, 350° C.)Steam (1 atm, 100° C.)2.0270.591.2Superheated Steam1.6233.956.4(10 atm 350° C.)Uranium0.12019,1001,292Granite0.7902,7002,133Molten Salt (142-540° C.)1.5601,6802,621Aluminum (#6061)1.2562,7103,404Cast Iron0.4567,9203,612Stainless Steel (Grade 316)0.5028,0274,030Sources:Water: http://www.engineeringtoolbox.com/water-thermal-properties-d_162.htmlSupercritical Water: www.isa.org/~birmi/magnetrol/Technical_Handbook.pdfSteam: http://www.thermexcel.com/english/tables/vap_eau.htmSuperheated Steam: http://www.spiraxsarco.com/esc/SH_Properties.aspxSalt/Metals: http://www.engineeringtoolbox.com/sensible-heat-storage-d_1217.htmlSteel: http://www.engineersedge.com/properties_of_metals.htm
Water can be superheated under pressure, and can have a boiling point as high as 374° C. under a pressure of 214 atm. Table I also shows the energy density achievable for water superheated to 350° C. If the production well is suitably airtight and pressurized, higher temperatures can be maintained, and with the greater temperature increase, significantly more heat can be pumped to the surface when superheated water is used. However, such high-pressure plumbing systems for a well several kilometers below the surface can be difficult to maintain. Also, superheated water can be a much better solvent for larger organic compounds, particularly if they have some polar groups or contain aromatic compounds, increasing the risk of contamination in the system. Therefore, superheated water can be more corrosive than water at ordinary temperatures, and at temperatures above 300° C. special corrosion resistant alloys may be required for the well casing, depending on the composition of the dissolved components.
An alternative to using superheated water is to allow the water underground to boil and become steam. Extreme pressures need not be maintained to control the flow of the steam at temperatures that can be significantly hotter than 100° C. But, as shown in Table I, the energy density of steam is significantly lower than liquid water. Even though the specific heat (2.027 kJ/(kg ° C.)) is smaller by only a factor of 2, the much lower density (typically 0.6 kg/m3) of normal steam means the same volume of steam holds 3,500 times less heat than liquid water. Supercritical heating of steam, increasing the temperature and pressure, can increase the volumetric energy density somewhat, but typically not by more than a factor of 10, and then the problems of managing an extremely hot fluid under pressure are reintroduced.
Table I also compares the energy density possible with water and steam with a few other materials, notably molten salt (heated above 142° C.) and several metals. These support an energy density much higher than that of steam for cases where the thermal pool is hotter than 100° C., especially for the case of stainless steel, where the energy density approaches water again.
There is therefore a need for a geothermal system which can operate as a closed loop system without causing seismic damage or groundwater contamination, but which also allows for a substance with a large volumetric energy density to be used to absorb heat inside the Earth from depths where the temperature is greater than 100° C., coupled with an efficient means to bring the heated substance to the surface of the Earth for thermal harvesting.